The present invention generally relates to processing steps of a master optical disc, and particularly to a processing method of a master optical disc, a stamper processed thereby, optical discs formed and manufactured with the stamper, and a processing method of grooves.
An optical disc is generally prepared by transcribing a concave/convex pattern by the injection-molding method etc. using a stamper provided with such concave/convex pattern as tracking guides (grooves) and pits. The method conventionally adopted as a means to model a concave/convex pattern of such stamper is exposing and recording a laser beam onto a master disc coated with photosensitive material. To the photosensitive material sensitized by the above exposure, a concave/convex pattern is formed by development and a stamper is thereby prepared by plating the surface thereof such that the concave/convex pattern is transcribed onto the metal surface.
FIG. 6 shows an optical path indicating a conventional processing method of a master optical disc. A laser beam 102 emitted from a laser device 101 is subjected to intensity modulation at an acousto-optical modulator 104 while reflecting at the mirrors 103, 105 and 107 and entering an objective lens 108. The modulating signal to be input to the acousto-optical modulator 104 is an input signal 601 turning on/off in accordance with the existence of a pit. The laser beam 102 transmitted through the objective lens 108 is converged at a master disc 109 coated with the photosensitive material and forms a spot 111 on the surface thereof. As a moving optical stand 106 provided with the objective lens 108 and mirror 107 gradually moves at a certain speed in the radial-direction of the master disc 109, the master disc 109 is loaded on a turntable 110 and turns simultaneously therewith. A spiral pit 112 is thus recorded on the master disc 109.
Regarding the shape of the pit 112, a short pit tends to be narrow and a long pit tends to be wide due to the interference of beams before and after the recording of the pit 112. Therefront, suggested are the method of recording compensation for changing the quantity of light in proportion to the pit length or the method of recording a long pit with intermittent beams as disclosed in Japanese Patent Laid-Open Publication No. HEI10-177722.
In case of employing the conventional method above to form a pit, recording with an input signal 501 as shown in FIG. 5 causes a problem in that the center portion 503 of a pit becomes wider than the front and rear edges 502 (hereinafter simply referred to as the xe2x80x98edgesxe2x80x99) of a pit due to the interference of beams and the characteristics of the resist. Unevenness in the width of a pit will cause adverse effects to the jitter and the modulation degree of a reproduced signal, and it will also lower the reading accuracy. Furthermore, the angle of the edge walls 504 of the pit becomes obtuse, and causes further aggravation of the jitter.
The present invention provides a processing method of a master optical disc by converging and exposing a laser beam to the master disc coated with photosensitive material, comprising an exposure method for forming one pit by exposing a laser beam composed of a plurality of pulse arrays with said laser beam being an intermittent beam, wherein the pulse length corresponding to the front edge of the pit and/or the pulse length corresponding to the rear edge of the pit is longer than the pulse length corresponding to the center portion of the pit. This processing method corrects the differences in width between the edges and the center portion of a pit, and it also makes the angle of the edges more acute. Therefore, it is possible to process a master optical disc having a fine jitter, a fine modulating degree and consequently fine reading accuracy.
The present invention provides a processing method of a master optical disc, wherein the pulse interval behind the pulse corresponding to the front edge of the pit and/or the pulse interval in front of the pulse corresponding to the rear edge of the pit is longer than the other pulse intervals of the pit. This processing method makes the angle of the edge walls 504 of a pit more acute and makes it possible to produce pits of a uniform width and further to operate a processing method of a master optical disc having a fine jitter.
Furthermore, the present invention provides a stamper manufactured by the aforementioned processing method. The present invention also provides optical discs manufactured using such stamper.
The present invention provides a processing method for forming grooves of a certain pattern by converging and exposing a laser beam to a master disc coated with photosensitive material, comprising a step for forming one groove by exposing a laser formed of a plurality of pulse arrays with said laser beam being an intermittent beam, wherein the pulse length corresponding to the front edge of one groove and/or the pulse length corresponding to the rear edge of said groove is longer than the pulse length corresponding to the center portion of said groove. This processing method corrects the differences in width between the edges and the center portion of a groove and also makes the angle at the edges more acute, and therefore it is possible to form grooves of high accuracy.
Furthermore, the present invention provides a processing method, wherein the pulse interval behind the pulse corresponding to the front edge of the groove and/or the pulse interval in front of the pulse corresponding to the rear edge of said groove is longer than the pulse interval of the center portion of said groove. This processing method makes the angle of the edge walls of the grooves more acute, and therefore it possible to form grooves of a uniform width and high accuracy.